For performing vacuuming, dry pumps have conventionally been used. The dry pump is provided with a pump chamber in which a rotor is contained in a cylinder. The dry pump performs vacuuming by rotating the rotor in the cylinder, and compressing and moving an exhaust gas so as to reduce the pressure of a sealed space at an intake port (for example, refer to Patent Document 1). Specifically, in a case where vacuuming is performed so as to obtain a medium vacuum or an excellent vacuum in the sealed space, a multiple-stage dry pump is used in which a center cylinder includes a plurality of pump chambers which are connected in series from the exhaust gas intake port to a discharge port (for example, refer to Patent Document 2).
When the dry pump is driven, the exhaust gas is compressed in the pump chamber and heat is generated, and the temperature of the cylinder thereby rises. For example, in a case where vacuuming is performed so as to obtain a general, preferable pressure by the multiple-stage dry pump, the inner pressure of a pump chamber provided near an air side (discharge side) becomes higher than the inner pressure of a pump chamber provided near a vacuum side. Accordingly, the amount of heat generation increases in the pump chamber provided at the air side.
A multiple-stage dry pump is well-known in which: an outer peripheral gas passage is cooled by a cooling liquid tank; and a counter flow port that can introduce a part of gas that flows through the outer peripheral gas passage into the pump chamber is formed (for example, refer to Patent Document 3). This method for cooling a pump chamber, which is a so-called counter flow cooling, can suppress the rising of the temperature of the pump chamber by introducing (flowing back) a part of gas cooled by the cooling liquid tank.